Although the sheet separating art is quite old, modern copiers, printers and scanners have required its perfection. The most critical sheet separating task in a copier or scanner is the document handler. In order to maintain the page sequential order of a stack of original document sheets, the sheets are commonly fed off the bottom of the stack. Because of the weight of the stack, bottom separation is more difficult than top separation. To further complicate the task, the copier is asked to handle used originals which vary in size, weight, age and condition. Not surprisingly, nearly all document handlers presently on the market come with instructions not to feed originals of unusual weight or of poor condition.
The predominant technology in high speed document handling involves separation of the bottom sheet from the stack with either a vacuum roller or a vacuum belt. These devices permit high speed feeding of a variety of documents without damage. However they generate noise which must be muffled, are expensive and require substantial power.
Other sheet separating tasks in copiers and printers have also become demanding. These apparatus are being asked to print onto a large variety of both paper and transparency stock. Copy sheet input mechanisms may feed a wide variety of sheets from as many as three or more sources and are asked to feed sheets hand supplied to the top of a stack of different weight sheets. Finishers often supply separator sheets and covers of greatly varying weight and texture. Duplex tray mechanisms are asked to feed a variety of sheets that may be poorly stacked and have toner images on one side that are easily smudged. Even though most of these applications involve feeding from the top of the stack, a much easier task than bottom feeding, vacuum feed is often used because of the variety of sheet types to be fed.
Scuff separating devices are much less expensive than vacuum devices, use less power and create less noise. They are commonly used in copiers and printers to feed blank copy sheets from the top of a stack. However, they have been considered too unreliable for separating sheets from a stack of used document sheets or from a stack of sheets having varying characteristics. They are especially difficult to apply to bottom feeding configurations. Prevention of double feeding and non-feeding in known scuff feeders is difficult with both such top and bottom separation applications. More important, because of reliance upon friction and a certain amount of slippage, they have been considered likely to damage more frail sheets.
U.S. Pat. Nos. 2,665,906 and 4,480,827 disclose top scuff separating devices in which the problems of slippage have been reduced by use of a braked retard roller opposite a larger drive roller. Although these structures have been in the literature for many years, to the best of applicants' knowledge, the principles have not been widely applied to the demands of the most difficult top or bottom separating problems. A possible reason why these structures have not seen substantial use is that each of them appears to require adjustment of the brake for extreme of paper weight and stack height. In the applications mentioned above, a variety of papers and stack heights is a normal condition.